Background: Decided to participate in a short ‘over the weekend’ data science hackathon. No aspirations of placing, just learning. What I’m presenting here is my interpretation of the question, and approach to the problem. (Unfortunately, there was some ambiguity as to the actual structure of the problem*)

Problem Statement

Basically a two-parter. Part A You, a data scientist at an insurance company, are trying to predict the probability of policy renewal for each customer (standard ‘churn’ problem, logistic regression). Part B was the optimization, and my main focus here: You can spend extra (Incentives) to have agents pay closer attention to clients, and thereby gain an Increase in Probability of renewal. This relationship is not linear, but rather defined the figure below.

pacman::p_load(tidyverse, plotROC, pROC, DMwR2, plotly)
fun.3 <- function(x) .2*(1-exp(-2*(1-exp(-x/400))))
p <- ggplot(data=data.frame(x=0), mapping = aes(x=x))
q <- p + stat_function(fun = fun.3)+xlim(0,1500)+ylim(0,.175)+
  xlab('Incentives Spent to Increase Renewal Probability')+
  ylab('Increase in Prob: delta_P')
ggplotly(q)

Terms

  • Incentive money spent to improve Probability of Renewal
  • Premium cost of the policy
  • P(baseline) model’s predictions of renewal probability, before incentives
  • delta_P improvement in Probability of Renewal from Incentive
  • P(incentivized) is the improved Probability of Renewal, after incentives, so P(baseline) + delta_P

The Goal

The optimization (Part B) is to maximize the following formula for Total Net Revenue, summed across all the rows of the dataset:

( P(incentivized) * Premium ) - Incentive

Basically, you are multipling the Premium (revenue), by your odds of receiving it (renewal probability), after accounting for costs (in this case, just the incentive to boost renewal probability). This is summed across all individual clients (rows in the dataset). Real life is more complicated, the goal here is a simple optimization toy problem.

My approach: Work out the relationships between variables using function approximation.

Considerations

  • Probability ranges from 0-1. If a customer is predicted to have a high probability of renewing their policy, no matter how much you spend on incentives, their renewal probability cannot be >1, by definition.
    • In other words, the maximum revenue per policy, before subtracting incentives, is the Premium.
  • You’ll want to maximize policies with high P(baseline) to 1, because small delta_Ps don’t cost very much (curve is steepest close to origin). It’s also the majority of cases, ~94%.
    • That seems silly and probably not something you’d do in real life, BUT the problem did not account for compensation of the agent, so your only goal is to maximize revenue according to the formula given.
  • The smaller the premium, the smaller the ‘upper limit’ will be on incentives: the increase in probability delta_P has a fixed cost, but the revenue boost it provides (delta_P * Premium) is a function of the Premium.
    • Introducing revenue-related term ROI, to differentiate: the ROI is the revenue-gain for applying the incentive, without consideration of P(baseline))
    • ROI formula: delta_P*Premium - Incentive
    • The optimum ROI will depend on the premium amount.

Scoring Metric

Scoring was a weighted average that favored (70% of score) the AUC for your Part A solution, with the remaining ~30% calculated from your total net revenue. I’ve included some code at the end to approximate this (solution checker page taken down after competition, boo)

EDA + Model

A simple logistic regression on untransformed variables performed fairly well in this case. (predictor variables included: age, number of premiums paid on time/late, income, and underwriter/credit score).
The classifier isn’t the focus here, so I’ll just show the quick and dirty code for Part A. The DMwR2 package in R makes kNN imputation quite easy - as there were a handful of NAs in both the training and test sets.

train <- read.csv('train.csv')
trainCC <- knnImputation(train[,-c(1)])
lr0 <- glm(renewal ~ ., data = trainCC, family = 'binomial')
lr0pred <- predict(lr0, trainCC, type = 'resp')
trainCC$lr0pred <- lr0pred
ggplot(trainCC, aes(d=renewal, m = lr0pred))+geom_roc()

roc <- roc(trainCC$renewal, lr0pred); auc<-roc$auc; auc # on training
## Area under the curve: 0.8317

Sometime soon I’ll make a companion post exploring how to optimize the binary classifier model. But since the best AUC I saw was ~0.85, I call that darn good for a first-pass. FWIW, feature engineering/transforms did little to enhance the model, according to several of the top-ranked participants, and my own fiddling (quite frustrating!).

Optimization Strategy

The first step was to figure out the optimum ‘ROI’ for Incentives, across the range of policy premiums. The smart way is to plot the equation, where z = ROI, x = Incentives, and y = Premium. It gives you a 3d surface - find the equation that defines the maximum for each x,y and you’re all set.

fun.3d <- function(x,y) (.2*(1-exp(-2*(1-exp(-x/400))))*y) - x

Except I don’t have the math skills to make that happen. The plot sure is pretty though…

incentives <- seq(0,1000,10)
premium <- seq(1200,60000, length.out=101)
roi <- as.matrix(read.csv('surface.csv', header = F))

plot_ly(x=~incentives, 
        y=~premium,
        z=~roi) %>% add_surface()

Instead I turned to (old faithful) Excel, and plotted a poor man’s 3d surface. We already know the relationship between Incentive and Increase in Probability delta_P, so we can multiply the latter by a range of premium amounts, and deduct the incentives paid to calculate the ROI (see below). Then you can use conditional formatting to easily spot the top 3 values per column.

The highlighted table in yellow is a list of values we can now fit to get a good approximation of that relationship. The output of that fit will tell us the optimal Incentive for each value of Premium. Note: ideal incentive maxes out at ~920, for the 60k upper limit on the range of policy Premiums in our dataset. Because of the asymptotic shape of that curve, it makes sense there is an upper limit, and that corresponds to an Percent Increase delta_P of 0.1669.

ideal <- read.csv('ideal.csv', header=T) # from excel
colnames(ideal) <- c('premium', 'incentive')
tail(ideal)
##    premium incentive
## 10   20000       580
## 11   25000       650
## 12   30000       700
## 13   40000       790
## 14   50000       860
## 15   60000       920
range(ideal$incentive)
## [1]  20 920
ggplot(ideal, aes(x=premium, y=incentive))+geom_point()

Now, this part will not make sense to most people - but since my background is in Pharmacology, I thought “gee that looks an awful lot like a dose response relationship.” (The top half of a sigmoidal curve going through the origin.) So, I downloaded the drc package to fit the data. I guess you’ll just have to trust me that it makes sense (and I’m sure there are other packages and formulas that would approximate the fit just fine). 

library(drc); library(modelr)
drc <- drm(incentive ~ premium, data = ideal, fct = LL.4(), type = 'continuous')
rsquare(drc, ideal)
## [1] 0.9997965
plot(drc, log = '', cex = 2)

Looks pretty close. Now we can call predict() to get the optimal Incentive as a function of Premium!

‘Room’ for Improvement

Now.. What about people who are already very likely to renew their policy? We used Excel and curve fitting to calculate the ideal maximum amount to spend on Incentive, as a function of Premium - above which our ROI starts to decline. But people with a low Probability of Renewal P(baseline) are a relatively small portion of the dataset (~6%). Performing well overall will require optimizing ROI from the majority who are already near P(baseline)=1.

Also noted from the Excel sheet: spending less than the optimal amount never resulted in a negative ROI. In other words, it is always worth spending a little to bump up your P(baseline) to 1, even if it was already very high.

Therefore, the first step on the test set is to calculate how much ‘room’ for improvement there is before you hit P(renewal) = 1. That is simply 1 - P(baseline) – which again is just the output of our logistic regression model.

test <- read.csv('test.csv') #read in
testCC <-  knnImputation(test[,-c(1)]) # don't use id for knn
testCC <- cbind(id = test$id, testCC) # add it back
testPred <- predict(lr0, testCC, type = 'response')  # get P(baseline) for test set
testCC$pred <- testPred  # add back to dataframe
testCC$room <- 1 - testCC$pred # calculate room for improvement

Solve for X (?!)

OK, next is where we run into a problem: We have a new maximum for how much we should incentivize (ideally), but in units of Y/delta_P (based on the relationship we already have for those variables; see fun.3 above). We want to calculate Incentive as a function of delta_P, when delta_P is dictated by ‘room.’

AND, the equation is ridiculously complicated, to the point that Wolfram Alpha can’t even solve for X. We could go back to school for a math degree, OR… use another function approximation from observable data points.

y = seq(0,1000, 10) # range of incentives, new Y
deltaP <- fun.3(y) # Increase Prob for range of incentives, new X
solveX <- data.frame(x=deltaP, y=y)
solveX <- solveX[2:101,] # get rid of origin, zeros
tail(solveX)
##             x    y
## 96  0.1673989  950
## 97  0.1675483  960
## 98  0.1676933  970
## 99  0.1678342  980
## 100 0.1679709  990
## 101 0.1681038 1000
plot((solveX$x), log(solveX$y))

So, it sort of looks like a 3rd order polynomial might approximate, but on closer inspection that doesn’t work very well. Instead, we’ll get rid of the values of delta_P/x above about .12, and try to use the dose response curve fit once again (we mostly care about the low values of delta_P that will dictate Incentive in cases where P(renewal) is already high).

solveX<- solveX[1:28,]
s <- ggplot(data=solveX, aes(x=x, y=y))+geom_point()+
  scale_y_log10(); s

solveX$log10incent <- log10(solveX$y)
drc2 <- drm(log10incent ~ x, data = solveX, fct = LL.4(), type = 'continuous')
rsquare(drc2, solveX)
## [1] 0.9977934
plot(drc2, log = '', cex = 2, xlab = 'delta_P')

Well, it’s not perfect, but R-squared is pretty good.

We’ll use this to calculate what the incentive should be, when the room for improvement is less than what the ideal incentive (only a function of premium) would suggest.

Here is the code to finish these calculations on the test set to get our predictions. If you followed along so far, this should be no problem.

testX <- testCC[,c(1,12:14)] # dplyr 'select' knitr fail

testX <- testX %>%
  mutate(ideal_incentive = predict(drc, testCC[c('id','premium')])) %>% # optimum based on premium
  mutate(ideal_delta_P = fun.3(ideal_incentive)) %>% # delta_P for ideal incentive
  mutate(actual_delta_P = pmin(room, ideal_delta_P)) %>%  # take lower
  mutate(p_incentives = pred + actual_delta_P) %>% 
  mutate(incen_room = 10^(predict(drc2, data.frame(x = room)))) %>% # remember y was log10(y)
  mutate(incentives = pmin(ideal_incentive, incen_room)) %>% # take the lower value
  mutate(new_prob = fun.3(incentives) + pred) # sanity check

head(testX, 10)
##       id premium      pred       room ideal_incentive ideal_delta_P
## 1    649    3300 0.9898966 0.01010340        180.2902     0.1031998
## 2  81136   11700 0.9787123 0.02128767        447.2463     0.1479551
## 3  70762   11700 0.9186891 0.08131092        447.2463     0.1479551
## 4  53935    5400 0.9701324 0.02986761        273.4742     0.1257213
## 5  15476    9600 0.9571893 0.04281073        399.5626     0.1434638
## 6  64797   11700 0.9776386 0.02236144        447.2463     0.1479551
## 7  67412    3300 0.5922917 0.40770834        180.2902     0.1031998
## 8  44241    5400 0.8720873 0.12791273        273.4742     0.1257213
## 9   5069   13800 0.9847190 0.01528100        488.7091     0.1511999
## 10 16615   28500 0.9881246 0.01187544        687.9440     0.1612744
##    actual_delta_P p_incentives incen_room incentives  new_prob
## 1      0.01010340    1.0000000   11.16315   11.16315 1.0006079
## 2      0.02128767    1.0000000   21.76264   21.76264 0.9988105
## 3      0.08131092    1.0000000  125.98737  125.98737 1.0021833
## 4      0.02986761    1.0000000   31.66498   31.66498 0.9983726
## 5      0.04281073    1.0000000   49.67713   49.67713 0.9988506
## 6      0.02236144    1.0000000   22.91513   22.91513 0.9987144
## 7      0.10319981    0.6954915 2119.29545  180.29016 0.6954915
## 8      0.12572131    0.9978086  265.09686  265.09686 0.9962045
## 9      0.01528100    1.0000000   15.76006   15.76006 0.9995907
## 10     0.01187544    1.0000000   12.67963   12.67963 1.0002239

Cool, check out line 8, where out new overall probability of renewal (p_incentives) is 0.9978. Is that just the error in our function approximations? Nope, it’s actually dictated by the ideal_incentive/ideal_delta_p, which is a hair less than what we have ‘room’ for (.125 vs .127). Neat, math is fun!
Line 7 on the other hand, started at ~.59 and had plenty of room – but, since the policy amount was low (3300), so was the ideal_incentive, so he ends up with a new renewal probability of only ~.69.

Evaluation

Of me: I got distracted as the deadline approached, and never submitted for a final score. I had made submissions that were on the top ~1/3 of the leaderboard before figuring out the optimization part.
Of the platform it’s a dud for people coming to learn:

  • doesn’t automatically submit your best score
  • the data, the problem decription, and solution checker are all taken down after the competition closes (I thought perhaps because it was a problem used for hiring/interviews, but its the same for all past competitions)
  • the ambiguity referenced earlier- there was actually a typo in the description of a variable, and the formulas given described increase in probability on a 0-100 scale. I’m sure most people caught it but just seems lazy.
  • No clarification of said ambiguity offered during the hackathon despite a Slack channel dedicated to it, and people asking for clarification.

A bit anti-climactic I suppose. Lesson learned: Stick to Kaggle if you’re learning.

If you have some optimization experience or math chops, I’d love to know the ‘right’ way to go about this problem, so if you have an opinion or alternate strategy, even a link to a useful tutorial, please share!

Here is some code to approximate your score on the test set (which is pretty easy to do, if you just use AUC on the training data)

# revenue without incentives, wi
# 0.9 threshold for renewal
wi <- sum(
  (ifelse(testX$pred>=.9, 1, 0) )*testX$premium) # 328M

# total possible revenue; sum of premiums
all <- sum(testX$premium) # 371M

# your revenue after incentives
rev <- sum((testX$p_incentives*testX$premium)-testX$incentives) # 366M

incent_score <- (rev-wi)/(all-wi); incent_score
## [1] 0.8384762
score = (.7 * auc) + .3*(incent_score); score
## [1] 0.8337112

Just a quick way to check your own ideas re: optimization. You would do a test-train split if you wanted to have a better idea of out-of-bag performance of your binary classifier.